Issue 50

L. He et alii, Frattura ed Integrità Strutturale, 50 (2019) 649-657; DOI: 10.3221/IGF-ESIS.50.55 651 Experimental procedures Microwave radiation experiments were performed in a commercial microwave muffle furnace (HAMiLab-M1500), operating at a frequency of 2.45 GHz, as shown in Fig. 2. According to the set temperature curve, the microwave system can automatically adjust and continuously output power in the 0.2–1.4kW range. The cavity was equipped with an infrared thermometer to measure the surface temperatures of the analysed specimen. Therefore, the temperatures described below refer to the surface temperatures of the analysed samples. It should be noted that according to the principle of microwave heating, the temperature inside the samples is higher than the surface. Considering the dimensions of the microwave cavity, the granite block was processed into cube-shaped samples (side length, 2.5 cm) and dried at 105 °C, until obtaining constant weight blocks. Rocks with no visible macro-cracks in six surfaces were selected as test specimens. Each set contained two specimens and only one was heated during each test. To avoid the high loss rate of heat, the specimens were placed into the cavity of alumina for thermal insulation. The temperature curve was set with the rate of increase of 10–20 °C per minute and was maintained for 15 minutes after reaching the 6 designated temperatures (300–800 °C with steps of 100 °C), following which the specimens were cooled to room temperature naturally in the microwave device. Figure 2 : Schematic of the microwave muffle furnace. Characterisation The thermal analysis was performed using a thermal analyser device (SDT Q 600, TA) to obtain the thermo-physical properties of the analysed granite. Thermogravimetric and differential scanning calorimetry (TG-DSC) was performed at temperatures ranging from room temperature (25 °C) to 800 °C in air atmosphere, and the heating rate was 10 °C/min. The microstructure analysis of the microwave-irradiated specimens was performed using a scanning electron microscope (SEM, EM5000, KYKY). Before and after the exposure to microwave radiation, X-ray diffraction (XRD) patterns of the analysed samples were obtained using an XRD analyser (X’Per PRO, PANalytical) to determine the changes in the composition and crystallisation. Operating conditions were: Cu Kα radiation (λ = 1.54184 Å), power 2.2 kW, scanning speed 15°/min, 2 θ ranging from 3° to 80°. Uniaxial compressive strength (UCS) was tested to evaluate the overall trend in the damage variation in microwave-irradiated specimens, using a compression testing device (TYE-300) at the loading speed of 0.3 kN/s. R ESULTS Thermal analysis ig. 3 shows the results of the TG-DSC measurements on Qingyuan granite. A slight weight loss was detected in the 25–500 °C range; this was attributed to the mineral dehydration or water evaporation [28,29]. Above 600 °C, a significant reduction of mass was associated with the process of dehydroxylation of biotite [30]. Above 700 °C, the weight of the sample was almost unchanged. Furthermore, the curve of the heat flow exhibited an endothermic peak at 573 °C owing to the α to β phase transition in quartz [9]. Appearance and microtopography Fig. 4 shows the microwave-irradiated specimens. The colour of granite became shallower with increasing temperature, which was attributed to the irreversible dehydration of Fe-rich minerals [31]. It can be seen in Figs. 4a and 4b that the granite specimen exhibited no appreciable change of appearance below 400 °C. Above 500 °C, cracking and melting were observed F